SEOUL (Reuters) - North Korea said on Tuesday its main nuclear complex was operating and it was working to improve the "quality and quantity" of its weapons which it could use against the United States at "any time."

The comments follow a declaration by the North in 2013 vowing to restart all nuclear facilities, including the main nuclear reactor in Yongbyon that had been shuttered.

It marked the first acknowledgement since then that the plant, which has been the source of fissile material used in the country's atomic weapons program, is operational, experts said.

"All the nuclear facilities in Yongbyon including the uranium enrichment plant and 5 MW graphite-moderated reactor were rearranged, changed or readjusted and they started normal operation," theNorth's state-run KCNA news agency said, quoting the director of its atomic agency.

"If the U.S. and other hostile forces persistently seek their reckless hostile policy towards the DPRK and behave mischievously, the DPRK is fully ready to cope with them with nuclear weapons any time," the director was quoted as saying.

DPRK is the abbreviation of the North's official name, the Democratic People's Republic of Korea. The country often makes threats against the United States and South Korea.

Innovations have been made to improve nuclear weapons "in quality and quantity as required by the prevailing situation," the director was quoted as saying.

Isolated and impoverished, North Korea's nuclear program is a key source of international leverage and a means of protecting the third-generation dictatorship of the Kim family.

The country is believed to be working toward developing an intercontinental ballistic missile mounted with an atomic warhead that can hit targets in the mainland United States.

Missile Test?

Late on Monday, the North's space agency said it was readying a new satellite for launch, indicating it may fire an upgraded long-range ballistic missile timed to around the 70th anniversary of its ruling party on Oct. 10.

Any such launch would violate international sanctions although the North insists it would be part of a space program for peaceful purposes.

North Korea is believed by experts to have enough fissile material to build 12 or more nuclear weapons, and is believed by U.S. and South Korean officials to be working to miniaturize a nuclear warhead to mount on a delivery vehicle.

China, the North's main ally, said on Tuesday it opposed nuclear weapons development on the Korean peninsula.

"We hope the relevant parties, under the current circumstances, can do more that is beneficial to safeguarding the peace and stability of the region and the peninsula," Foreign Ministry spokesman Hong Lei said in Beijing.

North Korea conducted three nuclear tests between 2006 and 2013, drawing international condemnation. China joined the U.N. Security Council in a sanctions resolution after the 2013 test.

Recent satellite imagery of the Yongbyon complex indicated new activity there, according to a report last week by 38 North, a website run by the U.S.-Korea Institute at Johns Hopkins University in Washington.

The activities could be part of work to produce new nuclear material that would be a step toward adding to the country's nuclear stockpile, 38 North said.

"It has been nearly three years since their last nuclear test and now may be the time for them to test theirevolved technology," said Park Jiyoung, an analyst at the Asan Institute for Policy Studies in Seoul.